Observation of Space Weather from the Galileo Medium Earth Orbit
Dyer, C¹; Ford, K¹; Morris, P¹; Taylor, B²; Underwood, C²; Rodgers, D³; Mandorlo, G³; Evans, H³; Daly, E³; Gatti, G^3
1QinetiQ; ²University of Surrey; ³ESA

The Galileo medium Earth orbit regime (MEO) is well suited to studying the dynamics of the Earth’s outer trapped electron belt since each satellite traverses the belt twice in every 14-hour orbit. Space radiation environment monitors are routinely operated on GPS satellites and are also planned to be flown on the IOV Galileo spacecraft. The long term presence of monitors, with the possibility for multi-point simultaneous measurements, is highly beneficial to the study of space weather and its interactions. This benefit is already illustrated by measurements from the GIOVE-A test satellite which carries two radiation monitors in order to carry out a characterisation of the MEO environment in advance of the main constellation. The monitors on-board GIOVE-A measure parameters of significance to engineering design of the full Galileo fleet, namely ionising dose and dose rate, charging currents due to electron deposition, proton fluxes >40MeV and ion linear energy transfer spectrum. These measurements also provide valuable information on the spatial, spectral and temporal profile of the belts and thus can be used to test models of key processes such as the acceleration of electrons to relativistic energy.

The Giove-A on-board radiation monitors have been operational since shortly after launch in December 2005 so approximately the last 10% of solar cycle 23 has been observed on the approach to solar minimum (depending on its exact date). Prominent in the observations over this time are many electron belt enhancement events (i.e. electrons accelerated to relativistic energies), seen via the large increases in charging rates and dose rates. These enhancement events are primarily due to persistent fast solar wind streams emanating from solar coronal holes and the consequential energisation of the outer belt. Such an environment is expected at this time since coronal holes continue to occur through solar minimum even though their peak frequency is in the declining phase of the cycle. A particularly large event in mid-April 2006 was associated with the improved efficiency of magnetic connection around the equinoxes. Also notable from the data is that a progressive hardening of the electron spectrum takes place during the course of each belt enhancement event. Another particularly large electron enhancement occurred in mid-December but had a different origin i.e. the arrival of a coronal mass ejection (CME) and associated geomagnetic disturbance. Again significant hardening of the spectrum was observed as the event progressed.

The December 2006 CME was associated with an active region on the Sun which released a number of flares. Two moderate solar particle events were observed via the on-board proton/ion detectors on Giove-A in the days just prior to the CME arrival. Total ionizing dose monitors showed that there was a major dose increment on the spacecraft around this time: this was not due to the particle events but to the electron belt enhancement following arrival of the CME. In fact, to date ionizing dose is seen to be delivered almost wholly during these periods of space-weather driven electron belt enhancement, with a very large fraction received in just a few of the events. Comparison of long-term total dose received with expectations from standard models is being carried out. Raw data indicates higher dose readings than might be predicted from standard models at high shielding depths. However great caution in interpretation is needed since accurate modeling of all aspects of radiation transport and sensor behavior is required. In addition we have only a short period of observation relative to the solar cycle so it may be some time before any final conclusions are reached. Nevertheless it emphasises the need for more monitoring.

Even given the relatively short duration of the Giove-A mission so far, the on-board observations are proving valuable to understanding space weather influences on the dynamic outer electron belt. This paper will outline the data available and the potential further applications.